Default settings are not very efficient, since they do not even reuse
connections.
And when each query requires connection you can have only ~80 QPS, while
by simply enabling connection reuse (connection_auto_close=false) you
can have ~500 QPS (and by increasing connection_pool_size you can have
better QPS throughput).
So this patch allows to pass through some connection related settings
for the StorageMySQL engine, like:
- connection_pool_size=16
- connection_max_tries=3
- connection_auto_close=true
v2: remove connection_pool_default_size
v3: remove num_tries_on_connection_loss
